1. Field of the Invention
This invention relates generally to image display apparatus and, in particular, to apparatus for scrolling displayed images obtained through a so-called character and pattern telephone access information network system (CAPTAIN).
2. Description of the Background
Systems have been proposed utilizing standard telephone network lines to transmit various kinds of image information and to display such image information on the picture screen of a television receiver. One such proposed system is the so-called CAPTAIN system.
In such CAPTAIN system the format of the displaying picture screen is formed of 204 dots in the column direction and 248 dots in the row direction. In describing such screen display the following definitions are generally employed.
(1) dot: the minimum unit for forming the picture screen;
(2) line: a series of 248 dots in the lateral or horizontal direction of the display picture screen that is used as the minimum unit indicating the display position in the longitudinal or vertical direction;
(3) sub-row: a number of display areas each of which is formed by dividing the display picture screen at its upper end by an area including 248 dots in the row direction and 12 dots in the column direction and which is used as a unit indicative of the display position in the column direction;
(4) sub-column: a number of display areas each of which is formed by dividing the display picture screen at its upper-left end by an area including 8 dots in the row direction and 204 dots in the column direction and which is used as a unit indicating the display position in the row direction;
(5) sub-block: a display area where the sub-column and the sub-row overlap that is used to specify the color; and
(6) picture screen header: the uppermost sub-row on which is displayed a title or the like concerning the information now being displayed, that is, the monitor display.
A typical display picture screen is formed of 17 sub-rows .times.204 lines and one sub-row is formed of 12 lines, however, in the following description, when the lines are counted from top to bottom, they are numbered from the 1st line to the 204th line, respectively, and when the lines are counted at every sub-row, they are numbered from the 1st to 12th line of each sub-row, respectively. Further, the sub-rows are numbered as the 0th sub-row to the 16th sub-row, respectively.
Generally, the format of the data signals, which are transmitted from the data base center of the CAPTAIN system to the terminal apparatus is chosen such that one section of the format is called a "packet". Each of these packets includes at its beginning a packet code indicating the kind of the packet it belongs to. Generally, there is a picture screen control packet, a color information packet, and a small character sequential display packet. Further, the picture screen control packet includes, following the packet code, a code indicative of the display mode and a code for designating the color of the picture screen header and the like. The color information packet includes a code indicating to which sub-row this packet belongs, or the display position per sub-row unit in the column direction and a color code for specifying the color of each sub-block in the sub-row designated by this code and so on. Additionally, the small character sequential display pattern packet includes a code indicating to which line this packet belongs, or indicating the display position of the line unit in the longitudinal direction and a pattern data indicative of dots on the line designated by this code. These packets are divided by flags each of which has a particular bit arrangement and then transmitted from the data base center of the CAPTAIN system to the terminal apparatus of the user.
The above-described picture screen control packet, can designate a fixed display mode and a scroll display mode, and in the scroll display mode, a difference between the display position of the displayed picture information and the display position of the newly received picture information is obtained and the picture information being displayed is shifted upwards by this difference amount, thus, the picture information is displayed under the condition that the new picture information is inserted into the lowermost portion of the display picture screen. The picture screen header is not displayed in the scroll display mode.
One kind of terminal apparatus for the CAPTAIN system is controlled by a microcomputer, having a central processing unit for parallel processing, a read only memory in which is stored its processing program, a random access memory for work area and buffer area, a video RAM having a capacity of one picture screen amount or above, a color picture tube, a read address control circuit, and a deflecting circuit. The output from the deflecting circuit is supplied to the color picture tube in which the deflection is carried out. The synchronizing signal from the deflecting circuit is supplied to the read address control circuit which then produces a read address signal corresponding to the deflection position of the picture tube, and this signal is also supplied to the video RAM. Accordingly, from the video RAM, there is read out address data corresponding to the deflection position of the color picture tube, and this data is supplied to the color picture tube which displays the data which is written in the video RAM.
Additionally, circuitry is required for connection to the telephone lines used to transmit the data.
Consequently, the data signal from the data base center of the CAPTAIN system fed through the telephone network lines is demodulated by a modulator/demodulator, converted from a serial signal to a parallel signal by a serial-to-parallel converting circuit and then fed to the CPU. Conversely, a data request signal from the CPU is converted from parallel to serial by the parallel-to-serial converting circuit, modulated by the modulator/demodulator and then fed through the telephone network line to the data base center of the CAPTAIN system.
When the above-mentioned scrolling display is carried out, the data access for the video RAM is generally carried out such that the video RAM is formed into one section in which the pattern data is accessed and one section in which the color code is accessed.
The read-outs for the video RAM sections are carried out at every field in synchronism with the scanning of the color picture tube. Wherein, the read address of the first section of the video RAM is varied at every horizontal period, while since the color is determined on a sub-block unit basis and one sub-block is formed of 12 lines, the read address of the second section of the video RAM is varied at every 12 horizontal lines.
Since the data transmission from the data base center of the CAPTAIN system and the scanning of the color picture tube are not synchronized with each other, the writing and the reading of data into and out of the video RAM sections are not always carried out alternately, and since the data transmission rate is lower than the display speed, the data is read out several times for one writing.
In short, upon scrolling display, the first to 12th addresses of the first video RAM section and the 0th address of the second video RAM section are used for the picture screen header and the data are written therein once. However, the last addresses or 216th and 17th addresses of the video RAM sections are connected to the 13th and first addresses thereof in an operation standpoint. Thus, the 13th to 216th addresses of the video RAM first section and the first to 17th addresses of the video RAM second section are formed as so-called ring shapes, respectively. Then, the data newly received are written in the next addresses (the address followed by the 216th and 17th addresses are 13th and 1st addresses) of the ring shapes. The scrolling display is generally carried out as mentioned above.
In this method, however, when the data of the color information packet or the small character sequential display pattern packet is not obtained due to noise and so on, mis-matching will occur between the pattern and the color which will hereinafter be scroll-displayed.
More particularly, since the color code and the pattern data of the 0th sub-row of the 1st page are the picture screen header and they are not scrolled, except the color code and the pattern data as described above, the beginning of each page becomes the 1st sub-row. For example, when the addresses of the video RAM sections in which the color code of the 1st sub-row and the pattern data of the 1st line are written are considered, they are written in the 17th and 205th addresses with respect to the 1st page, while they are written in the 16th and 193rd addresses with respect to the 2nd page, and they are written in the 15th and 181st addresses with respect to the 3rd page. In other words, if the page is changed, even with the same sub-row and the same line, the addresses in which the color code and the pattern data thereof are written are decremented at every page by 12 addresses and 1 address, respectively.
Accordingly, even if the color information packet and the small character sequential display pattern packet contain the codes indicative of their display positions, the addresses in which the data are written are changed with the pages so that it is very difficult to write the color code or the pattern data in the two sections of the video RAM by using the display position codes.
To cope with this defect, when the color code or the pattern data is obtained, this color code or the pattern data is written in the address following the address in which the previous color code or pattern data is written.
Accordingly, if the color code of the color information packet at its n-th address is not obtained due to, for example, noise, the color code of the color information packet at its (n+1)th address is written in the address in which the color code of the color information packet at the n-th address should be written. Thus, thereafter, all color codes are written in the video RAM with addresses decremented by every one address (the color code of one sub-row amount is displaced upward in the picture screen).
As a result, if the color code and the pattern data are read out to thereby carry out the scroll display, all the pictures under the sub-row of which the color code can not be obtained are scroll-displayed with the color being displaced by one sub-row amount relative to the pattern data, and this is continued until the scrolling display is ended.
On the other hand, when the pattern data of the small character sequential display pattern packet is not obtained, due to the similar reason, the succeeding pattern data is incremented by one address and then written in the video RAM hereinafter. Consequently, all picture images below the line of which the pattern data is not obtained are scroll-displayed such that the patterns and the colors thereof are mis-matched by one line each. This is continued until the scrolling display is ended.
As described above, if the color code of the color information packet and/or the pattern data of the small character sequential display pattern packet are not obtained, in the following scrolling display the pattern and its color are all displaced with each other.